During the last fifteen years, various antiviral chemotherapeutic agents have been developed for clinical evaluation. A problem with the development of such agents is that, unlike bacteria, viruses are not free-living organisms and are dependent for replication on the life processes of the host cell which they are infecting. It is therefore highly desirable for the antiviral agent to exert its effect specifically on the replicative processes of the virus rather than on the corresponding processes of normal (non-infected) cells. The antiviral agents so far developed act via a variety of mechanisms to exert their antiviral effects. These mechanisms involve inhibition of different stages in the process of viral replication in the host cells.
One particular stage of replication at which the virus is susceptible to inhibition is the stage of nucleic acid replication, i.e., the production of DNA from DNA, RNA from RNA, or DNA from RNA (depending on whether the virus is a DNA or an RNA virus), where the viral DNA or RNA acts as a template for the production of new DNA or RNA. In the case of DNA viruses, the production of new viral DNA involves the interaction of the enzyme DNA polymerase with the constituent nucleotides (specifically deoxyribonucleotides) which act as building blocks for the new DNA. Antiviral action at this stage generally involves the use of "fraudulent" or deleterious nucleotides which mimic the normal viral materials and either compete for DNA polymerase and/or are incorporated into the viral DNA chain to make it non-functional.
These "fraudulent" or deleterious nucleotides comprise a triphosphate derived from a nucleoside analog which is converted by enzymes first into the monophosphate and then subsequently into the diphosphate and finally into the triphosphate. An example of this type of antiviral agent is the marketed compound, acyclovir, i.e., 9-(2-hydroxyethoxymethyl)guanine (U.S. Pat. No. 4,199,574), which contains an acyclic side-chain in the 9-position of guanine compared with a cyclic sugar residue in this position in guanosine. The antiviral mechanism of action of acyclovir is believed to involve first its conversion to acyclovir monophosphate by the enzyme thymidine kinase, which is specific to herpes-infected cells. Once formed, acyclovir monophosphate is converted by normal cellular enzymes (kinases) via the diphosphate to acyclovir triphosphate (ACV-TP). Acyclovir triphosphate is believed to serve as an inhibitor of viral DNA polymerase since it resembles the natural nucleotide substrate, deoxyguanosine triphosphate (dGTP), and as a result competes with dGTP for binding to the DNA polymerase and thus competitively inhibits the effectiveness of the enzyme and consequently viral replication. When ACV-TP acts as a substrate for DNA polymerase it becomes incorporated into the viral DNA chain, but since it lacks the 3'-hydroxyl group present on the cyclic sugar moiety of the natural nucleotide substrate, it presumably acts as a DNA chain terminator. It also apparently inactivates the viral DNA polymerase. Thus, viral replication is prevented.
The antiviral effect of acyclovir, and related compounds which operate via an analogous mode of action, is believed to involve competitive inhibition, apparent inactivation of the viral DNA polymerase, and termination of the growing DNA chain.
A disadvantageous aspect of a competitive inhibitor is that the normal substrate may accumulate and become more effective in competitively blocking the binding of the inhibitor. In this manner, the build up of, for example, dGTP may hinder the binding of ACV-TP to the polymerase and thereby prevent subsequent inhibition and termination of viral DNA processing.
European Patent Specification 135,713 (U.S. Pat. No. 4,758,572) discloses antiviral combinations of nucleoside analogs and thiosemicarbazone ribonucleotide reductase inhibitors. U.S. Pat. No. 4,719,221 discloses the thiosemicarbazones in European Patent Specification 135,713.